Systems for treating exhaust gas produced by an internal combustion engine to reduce exhaust gas emissions in the form of particulates and/or NOx are generally known. Such exhaust gas “aftertreatment” systems may include a selective catalytic reduction system for the purpose of reducing the NOx level of the exhaust gas below a specified maximum NOx level.
A selective catalytic reduction system, or SCR, typically includes a catalytic reducing agent, or reagent, in the form of a reagent solution that is controllably dispensed into the exhaust gas stream produced by the engine before entering a conventional SCR catalyst. The SCR catalyst reacts with the combination of engine exhaust gas and reagent solution in a known manner to reduce the NOx content of the exhaust gas stream.
Prior to dispensing the reagent solution into the exhaust gas stream, there may be opportunities for the solution to become under-concentrated, over-concentrated and/or exposed to contaminants as a result of manufacturing concentration variability, tampering, accidental introduction of improper fluid, mechanical failures or undesirable mechanical operating conditions, and/or environmental degradation. It is therefore desirable to monitor the quality, e.g., concentration and/or contamination level, of the reagent solution under a wide variety of operating conditions, and to note occasions when the reagent solution quality is not within specified reagent quality limits, and to further modify conventional dosing of the reagent solution into the exhaust gas stream when the temperature of the reagent solution is at, or near, freezing temperatures.